CHAPTER 2 LITERATURE REVIEW

This chapter reviews the evolution of dielectric resonator antenna

technology over the past 24 years from the first cylindrical DRA to the latest magneto

DRA. An account of the existing design aspects for achieving wideband operation, multi­

band operation, conical radiation patterns, circular polarisation and compactness for

DRAs has been presented. Finally, a survey of the FDTD analysis of DRAs has been

carried out.

2.1 DIELECTRIC RESONATOR ANTENNA - THE BEGINNING

Dielectric resonator (DR) is a ceramic puck characterised by a definite

volume, shape, high dielectric constant and low loss. Radiation from open DRs

was realized by Richtmyer in 1939 But the first theoretical and experimental

analysis of a cylindrical DR antenna was carried out by Long er a/. in 1983

Since then, DRAs transformed into a fast growing focus among the antenna

researchers so that new DR geometries like rectangle, hemispherical, triangular,

ring etc. were evolved and studied extensively [3-7],

Kishk et a/. presented a detailed numerical analysis of a dielectric disk

antenna placed on a conducting surface based on surface integral equations to

compute the resonant frequency and Q-factor Later, Mongia at a/. published a

comprehensive review of the modes and radiation properties of various DRs [10].

Accurate closed-form expressions for the resonant frequency, radiation Q-factors

and the inside fields of a cylindrical DR were also described in the above work.Z3

.............. »»»» .....................A detailed study of the fabrication imperfections of probe fed cylindrical

DRAs was reported in [12]. It described that even thin air gaps between the DRA

and the ground plane backing or between the probe and the DRA can notably

modify its input impedance. An account of the various technological aspects of

DRA research at the Communications Research Centre, Ottawa, Canada till year

1998 was published in [13]. This included novel DRA elements and arrays

designed for wide bandwidth, compactness, circular polarisation, high gain and

active antenna applications. All the existing feed mechanisms such as coaxial

probe [2, 8], microstrip [5, 14, 15], coplanar waveguide [16, 17], slot-line [6, 11,

18], waveguide [19-21] etc are compatible with DRAS also.

2.2 WIDEBAND DRAs

In the early 90$ when junker at a/. noted that the presence of an air gap

between a cylindrical DRA and the ground plane, a kind of fabrication

imperfection can cause broadening of the resonant curve of the DRA [12, 22]. .

This was the effect of reduced unloaded or radiation Q~factor (Q11 or Q,) of the

DRA due to an increased effective radiating area. Later, Ittipiboon ez‘ a/.

introduced an aperture fed rectangular DRA, with its centre portion removed.

This DRA and its image formed a rectangular ring DRA to acquire a 28 %

bandwidth [23]. This was motivated by the work of Verplanken ez‘ a/. [24] which

reported that the Q, of certain modes of a cylindrical ring DR is lower than those

of the corresponding cylindrical DR.

24

...................... %%%%%%%%%%%%%%% (((((Use of multiple DR_As as part of the radiator provided an easy and straight

forward way of exciting multiple radiating modes, which facilitated multi-band

DRA design. By properly choosing the er and dimension of individual DR_As, the

resonant frequencies corresponding to the modes can be brought closer so as to

enhance the bandwidth. Kishk ez‘ a/. showed that a low er DRA stacked on top of a

high at DRA could provide a bandwidth of 25 % [25]. Further studies on stacked

DRA designs were carried out both experimentally and numerically [26-28]. Two

rectangular DRAs separated by a metallic plate yielded a much broader bandwidth

of 76.8 °/o [29]. Keeping the separate DR elements as a single entity in the above

cases was tiresome and was avoided by fabricating single stacked DRA structures

in the form of flipped staired pyramid [30], L and T shaped equilateral triangular

[31, 32] which offered a maximum bandwidth in excess of 60%.

Special DRA shapes like split-cylinder [33] and conical [34] were also

reported to have wide bandwidths. Such geometries however suffered from an

increased antenna dimension, especially the DRA height, compared to an

individual element. Embedding one DRA within another, in the form of an

annular ring solved the above problem where the antenna dimensions are the

same as that of the parent DRA [35, 36]. Later, a stacked-embedded DRA design

improved the bandwidth to 68 °/o [37]. A detailed comparative study of the

stacked and embedded wideband DRAs with the homogeneous DRA was also

carried out [38].

25

Literature Review_-_-_-_¢_t~_--é—_--.7-_=—__A_-_ ——————————— -- —-»-- ~ ~ ——~--—— —~-,----..:-:»_»__-_-;—_¢_—__'_§---i;__'_---».¢...~--~- _—;_-;_—_:—_—_-_—_---.-—-——- ——__——__—_—___—_...-.,------—;-1.-_-_—_—_-v.--.-.. —— —— ——___ —¢ '__—_-_—------~~—_-__—— _——___-__—_—_ .41»-~L—_——__— —__—___—_—_~-.---~ _—__—— ;—---\ — _ ——

Modification of the feed geometry proved to be a successful method for

improving the impedance matching and bandwidth. Luk er a/. used a vertical

metallic stub extended from a microstripline [39] or a coaxial probe [40]

enhanced the bandwidth to 19 % and 43 % respectively for cylindrical and

rectangular DRAs. In addition, this was also shown to improve the impedance

matching. A fork-like tuning stub [41] coupling energy from a microstrip through

a circular aperture to the DRA also improved the bandwidth. Feeding techniques

like L-shaped [42] and T shaped [43] microsttip also improved the impedance

bandwidth. To be suitable with low—Q, DRA shapes like cylindrical cup, novel

feeds like L, hook and] shaped probes [44, 45] were also found suitable in

addition to the probe or slot feed. An aperture feed which excites the DRA in

addition to radiating itself [46, 47] was capable of producing two merged­

resonances causing wide bandwidth operation.

Designs using a simple DRA is also presented for bandwidth enhancement

[48, 49], where an aspect ratio greater that unity allowed excitation and merging of

dual modes of similar radiation properties.

2.3 MULTI-BAND DRAs

A dual-band antenna can replace two single band antennas of suitable

operating bands. The work [25] on stacked wideband DRA was an implication to

the design of dual-band DRAs by choosing two DRAs of different dimensions,

excited by a single feed.

26

....................A wideband antenna unless is operating over a useful application band, is

useless. This suggests the design of independent application bands where the

antenna radiates only over those bands. Z. Fan at al. introduced a slot excited

double element rectangular DRA for dual or wideband application [50]. Stacking

of two cylindrical DRAs excited by an annular ring excited by a probe has shown

a three—band behavior [51]. In [52] dual frequency operation was achieved by

incorporating additional DRA in a parent DRA, both cylindrical in shape, so that

the volume of the structure remains unchanged. A cylindrical ring DRA is fed

with two orthogonal microstrip feeds is reported [53] for dual resonance. This

also has the effect of producing orthogonally polarised bands but with similar

broadside radiation patterns. Special eye—shaped DRA is also shown to be

effective in producing dual radiating modes [54].

By adding an additional radiator to the DRA also, dual-frequency operation

can be achieved. This principle is implemented in [55] where a cylindrical DRA

and a ring—slot are fed together by a circular slot thereby allowing radiation from

the two at respective resonances. If the feed to the DRA is also radiating at a

particular frequency, it will be advantageous in this context. This technique is

explained in [56] where the rectangular slot-feed to the DRA is made radiating by

adjusting its dimensions. The same team introduced another design [57] by using

a T-shaped microstrip feed that radiates in addition to exciting the DRA. A

ceramic loaded annular ring monopole antenna is found to resonate in the dual

W/LAN bands [58]. The principle is nothing but the inherent size reduction

property of high at DRs to be explained in the next chapter.

27

2.4 MONOPOLE/CONICAL BEAM DRAs

Most of the DRA designs discussed above produce broadside radiation

where the pattern maximum occurs at the broadside or zenith (6 = 90°). This is

achieved by properly exciting the magnetic dipole mode of the DRA [10]. But

certain other applications like high performance radio local area network

(HIPERLAN) [59] and vehicular communications need monopole-like or

conical radiation patterns with the maximum radiation located at the elevation,

most commonly in the range 30° < 6 < 60° and a null at the zenith. This is done in

most cases by exciting the monopole-like modes of the DR with a center-fed

coaxial probe.

In 1993, Kishk et a/. noted that in the case of a probe fed cylindrical DRA,

when the probe is positioned close to the centre of the antenna, a monopole

TMOn]8 mode is also excited with the same strength as the broadside HEM1m5

Mongia er a/. demonstrated that a cylindrical ring DRA excited in the TM015 mode

can produce near mon0pole—like radiation [9] over a small bandwidth. Later, this

design was modified for size reduction and good mode separation by

incorporating a metallic cylinder concentrically inside the ring DRA [61].

A combination of microstrip line and probe was used for exciting

monopole mode of a cylindrical ring DRA [62]. Conical beam operation is also

realized by the use of two cylindrical DRAs fed by a single probe [63]. This has

the effect of a 35 % increase in antenna gain but a 0.7 % decrease in bandwidth

compared to a single element.

28

_____-1.....a.....a.u.a.a._».....+.._.......,......._...a...,.-.r................--.....................,a.._.-.,.._._.._ .... -.t_-.-......._..._......._.-.............- ..... ....... ...- ____ _..._.............-...-..--.-.. ((((((((((((((((((((((( .

Recently, Guha ez‘ a/.' showed that by using an array of four cylindrical

DRAs surrounding a coaxial probe [64] a wider impedance bandwidth of 29 %

can be achieved. Special geometries like stacked triangular DRA [32] or half­

hernispherical DRAs [65] have also been reported to provide wideband conical­

beam operation. Ultra wideband monopole mode DRA was employed

[66] by combining a quarter wave monopole and a ring DRA. The design details

of the above antenna were given by Guha et a/. [67]. In this, three resonances,

resulted from the bare monopole, the ring DRA and the DR loaded effective

monopole, combine to provide the UWB response.

2.5 CIRCULARLY POLARISED DRAs

Generally a DRA produce linearly polarised radiation when operated on

any of the fundamental modes. The ability of DRAs to support multiple radiating

modes simultaneously is well exploited in the design of circularly polarised (CP)

antennas.

CP antenna design using DRAs started with [68] that propose a

rectangular DR with two diagonally opposite corners truncated similar to the

design implemented with rectangular patch antennas to produce CP. Later,

Mongia er a/. [69] produced CP by exciting the two orthogonal HE115 modes of a

cylindrical ring DRA using a 3dB quadrature coupler. In [70] the orthogonal

HE115 modes of a cylindrical dielectric resonator are excited by two probes fed in

phase quadrature by a microstrip line. In [71] a slot-coupled rectangular DRA is

used where the DRA position is adjusted 45° with respect to the slot to produce

29

§_if@fa£¥".€.K@"i@W---—~-=1»—1:.-_-.,~--_-1--_-_-_'_'_»_-__-_—;. -.,-..-n-:-_-;-_;'_-_-__'.—....; ------- --.--_- ----- --———- —— —~»- -— — \.---——-~ ;:-.1.-_—_—.—----.----———~ ———~--—— --.--u -——-~ —i.—._'...—_-\----— -;-_—i.—:.'.—.-.v--- ~ ~ — —— —~-—-~_'- ——;;:___—.-Ar-.--7; ~ _—:'.'...—~r--tr ~::_—

CP from a cylindrical DRA over a bandwidth of 3.4 °/o and beam width of 110°. A

cross-slot of unequal slot lengths in the ground plane of a microstrip line has been

used [72] to produce CP from a cylindrical DRA over 3.91 % of bandwidth. A

design with dual conformal strip feed [73] can also produce CP but over a wide

bandwidth of 20 °/o. A cylindrical DRA is fed by a perturbed annular ring slot [74]

to achieve an axial ratio bandwidth of 3.4 %. In this design, a backing cavity of

hemispherical shape was used to block the back lobe radiation. Effect of parasitic

conducting strip loading on the impedance characteristics of a cylindrical DRA

has been studied [75] where CP is achieved by varying the angular position of the

parasitic strip relative to the conformal feed.

A design similar to [71] is reported [76] where a parasitic strip is diagonally

attached on top of a rectangular DRA and is fed through an aperture. The design

is also shown suitable for a four element sequentially rotation DRA array. CP

from a hemispherical DRA [77] has been produced by using an approach

discussed in [75]. In the design, a slot aperture is used for coupling and the dual

orthogonal modes for CP are generated by using a grounded parasitic strip

attached to the DRA surface. A new geometry DRA in the form of a stair has

been fed by a slot is also reported [78] for generating CP over a 10.6 %

bandwidth. In [79] a 4.7 °/o axial ratio bandwidth has been achieved by using a

cross-slot of unequal slot lengths for coupling to a cylindrical DRA. A cylindrical

DRA with longitudinal slots of limited depth has been shown to produce CP over

a 4 % bandwidth [80]. Compact CP design [88] is also reported using a half-split

cylindrical DRA.

30

¢,~,-4-—--¢---0---------Q‘-1----4-h»--‘ow»-q.~....~----.,-..--¢-I-5------V-u-..~~u<n-v—_-_=_ ._..__.__..—;=—.==—.-.=—;==—;;;': ———~:. ~ — ------------------------------------------------ -.,.-.-B~~-~---\.~._\....4.

2.6 COMPACT DRAs

Design of compact DRAs has always been a challenging issue among

antenna researchers. By using a high dielectric constant material, a small volume

of DR can resonate at a lower resonant frequency as per Eq. (3.8) in Chapter 3.

But this will increase the Q-factor and hence a lower bandwidth with the resonant

frequency becoming highly temperature dependent [81].

Mongia er al. noted the existence of one or more planes of symmetry for

isolated DR shapes. \X7hile this plane of symmetry serves as an electric wall for

certain modes, it behaves as a magnetic wall for the other modes. This was the

motivation for the work [82] where a half-split cylindrical DR was paced over a

metallic ground plane, which was at the plane of symmetry (£15 = O). The particular

antenna configuration was excited in T E015 (magnetic dipole) mode with a low Q,

thereby facilitating more than 8 °/o bandwidth. A similar split-DR with a slot feed

was used for allowing integration with MICs [83]. Numerical analysis of a half­

split cylindrical DRA on a ground plane excited in the low Q, modes -TE015 and

HEM125 is presented [84] using a method of moment approach for the coupling

between a body of revolution (BOR) geometry and a non-BOR geometry.

M. T. K. Tam et a/. reported a half-volume design [85] for the broadside

modes of a cylindrical (HEM115) and rectangular (TE115) DRAs based on the

aforesaid approach. But they used an additional metallic plate attached to the

plane of symmetry of the DR which was oriented in the orthogonal plane of that

in [82]. In the same paper, the above team put forward the thread for further size

31

é.飧zi9wr@R¢vi¢w--.-- W ?????? .... mm __ _______ _ __e——_—ee;—;—e—_T——e—_:—_e_—_e_-_,-_-_.-_=_:_:_—-¢-—_¢_~=--a ' ~ ' ' ~ ' '- ~ ~ :_;—_—:;—_——:.~.~::<:::;:

reduction of the DRA by using a metallic post instead of the metallic plate. The

FDTD analysis of the above design was carried out by Steven G. O’Keefe [86]

additionally demonstrating a higher directivity for the half-volume DRA.

But a revolutionizing low-volume design was presented by Tam at a/. in

1999 using circular and annular sector DRAs [87] where a 75 °/o reduction in

volume is demonstrated. The design used different inner to outer radius ratios,

sector angles and boundary conditions (metallic, open or mixed) for the sector

DRA. A modification of the structure in [85] was used to produce circular

polarisation [88]. Kishk ez‘ a/. exhaustively studied the bandwidth enhancement

property of split-cylinder DRAs numerically and experimentally [89]. Use of

partial vertical and horizontal metallizations on a rectangular DRA has been

proposed to reduce the overall dimensions of the DRA to be used at WLAN

applications [90]. A thorough analysis of a reduced volume rectangular DRA

based on the above principle has been presented [98] using FDTD and

measurements.

For the enhanced miniaturization and a simultaneous increase in

bandwidth, the new DRA trend was introduced by K. Sarabandi at a/. utilising

magneto—dielectric materials having both relative permittivity and permeability

greater than unity [91]. The dimensions of the DR is thus proportional to the

square root of the product of at and pr. Recently, the experimental and theoretical

aspects of a probe fed cylindrical DRA based on a magnetodielectric material have

been studied [92].

32

............. iiiiiiiiiiiiiiiiiiiiiiiiiiiiiii %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% WQl?QPl§.C_?.

2.7 FDTD ANALYSIS OF DRAs

Tl;

Modeling of DR based structures using the FDTD method has been a

major research area during the past few years. In 1991, Navarro et a/. [93]

theoretically obtained the resonant frequencies of a cylindrical DR enclosed in a

metallic cavity, using FDTD combined with discrete Fourier transform

Kaneda at a/. presented a modified contour-path integral FDTD [94] to analyze a

shielded cylindrical DR, while maintaining the rectangular cells. In [95] Dey er a/.

discuss a conformal FDTD approach for cylindrical DR modeling based on

weighted volume effective dielectric constant. An alternate and easier method was

proposed by Yu er a/. [96] based on a linear average effective dielectric constant

concept. A fast and more accurate computation of resonant properties of

axisymmetric DRs using FDTD was presented by Shi ez‘ a/., by using the Pade'­

DFT technique [97].

Much works on the modeling and analysis of DRAS using FDTD are not

available in the literature. In 1994, Shum er al. studied the effect of an air gap

between the DRA and the ground plane on the resonant frequency of a coaxial

fed cylindrical ring DRA using FDTD [98]. The FDTD coordinate system used

was cylindrical (r, Q5 and Z) because of the axial symmetry of the structure. An

absorbing boundary condition (ABC) based on a parabolic interpolation was used

to tenninate the boundary and was placed at distances three times the dimensions

of the DRA. A Gaussian-modulated sinusoidal pulse was used for excitation. The

same team also calculated the resonant frequency of an aperture coupled

rectangular DRA using FDTD [99].33

Li€.¢r2£.4r.¢1r1§,§1..\2.i.q_*:'.___.r.rrr. .............. -.__..,.,,,.m.. .................... ...... __...........,.r...-..-._..-.._...

Later, Esselle [100] obtained the radiation patterns of an aperture coupled

low-profile rectangular DRA using FDTD. Mur’s ABC was used to terminate the

volume. The patterns were obtained by using the equivalence principle over a

fictitious surface enclosing the DRA.

But the fundamental probe fed cylindrical DRA structure of [2] was

analyzed by S. M. Shum at al in 1995 [101] and in detail in 1998 [102]. Since their

DRA is offset-fed, unlike the approach in [98], the rectangular coordinate system

was used with the second order Liao’s ABC at the boundary. Various feed models

like Voltage gap, jenson and Magnetic-frill for the coaxial probe have been

compared. A simple Gaussian pulse excited the system. The equivalent sources

just above the DRA are calculated by using the Goertzel algorithm and the far­

fields are computed by using the fundamental integral equations, The effect of

fabrication imperfections of the DRA on its performance has also been simulated.

The effect of ground plane thickness and coupling slot geometries on the

field coupling to a cylindrical DRA has been studied using FDTD by Guo ez‘ a/.

[103]. Four layer perfectly matched layer (PML) ABC has been used. In 2002,

O’Keefe et a/. studied the radiation characteristics of reduced-size DRAs [86]

using FDTD. A four layer PML ABC with parabolic conductivity profile and

Gaussian pulse excitation were used. In the work, they also computed the return

loss, in-field and radiation patterns of HEM118 , TE11s and TEW; modes.

Conformal FDTD was used by Farahat ez‘ al. [104] to analyze a circularly

polarised cross-shaped DRA. It is shown that the conformal mapping offers a 2:1

34

iiiiiiiiiiiiiadvantage over staircase mapping, without compromising accuracy. FDTD was

used to design the dimensions of a rectangular DRA, feed and parasitic elements

for the design of a CP DR.A [88]. Sernouchkina ez‘ a/. presented a detailed analysis

of the modes in a rectangular DRA fed by a microstrip line [I05]. Additionally,

the influence of DR dimensions, feed location and surface metallization on the

modes was also presented. In [I06], Lan et a/. used FDTD to design both the

radiator and feed sections a combination antenna using a rectangular DR_A and an

inverted L-plate. A complemented dispersive boundary condition has been

implemented in order to reduce the computational domain.

In this chapter, a detailed survey of the past works carried out in the field

of dielectric resonator antennas was presented. Various aspects such as bandwidth

enhancement, multi—band operation, circular polarisation, pattern modification

and compact designs for DRAs were discussed. In view of the above, a new DRA

design for wideband conical beam applications is proposed and studied in the

forthcoming chapters.

35

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